The field of injection molded thermoplastic optical disks includes all products using laser-read digital and/or analog encoded information stored on such molded disks. For example only, such disks can be as large as 300 mm diameter video disks recorded with a feature length movie, or as small as a 89 mm magneto optical erasable memory disk for computer storage of several hundred megabytes of data. However, the most popular is the 120 mm size digital audio compact disk for music, of which current worldwide production exceeds 400,000,000 CD per year. Drastic improvements in productivity have been made in the past 3-4 years, with molding cycles of 12-15 seconds being now dropped to about half that, with corresponding increases in throughput. However, further improvements are both desirable and necessary.
The molding cycle time can be broken down into the following stages:
1. Fill and pack PA0 2. Cooling until solidified PA0 3. Removal of solidified disk from the moldset
In analyzing the constituent parts of CD molding cycle time, steps #1 plus #2 would equal the "mold closed" time and step #3 is the "mold open" time. Possible improvements in any one of these steps may reduce total cycle time; for example, Applicants' Ser. No. 07/355,754 filed May 22, 1989, now U.S. Pat. No. 4,918,634, issued Jan. 1, 1991, teaches a faster, cleaner, and simpler way to remove the molded disk from the moldset, thus reducing "mold open" cycle time.
However, in the current CD molding process state of art, fill and pack typically take only a half second and disk removal time is less than 2 seconds, so about two-thirds or more of the total cycle time is due to cooling of the molten plastic in the molded disk and its sprue. Applicants' U.S. Pat. No. 4,793,953 teaches a way to improve heat transfer and shorten cooling time of the molded disk itself between two optically polished part forming surfaces of the disk moldset. However, the cooling time of the disk itself may actually be shorter than the required minimum cooling time of the sprue, in which case the sprue's cooling time dictates the minimum total cycle time of the molding operation. Improvements in this sprue cooling, therefore, are extremely desirable, since it appears to be the "bottleneck function" in the current state of arts. It is the subject of the present invention.
The purpose of the sprue in an optical disk moldset is to provide a passageway for the injected molten thermoplastic to travel from the nozzle tip of the injection molding machine into the disk mold cavity itself. The sprue must therefore provide a desirably large aperture through which the melt will flow with minimum impedance during fill (constrictiveness in this stage will result in slower filling time and both cosmetic and molded-in stress problems in the resulting molded disk). This sprue aperture size also determines the amount of pressure drop incurred between the nozzle tip and the gate into the disk mold cavity itself (too high a pressure drop during the pack stage can give inadequate packing to the melt in the disk mold cavity, resulting in poor surface replacation and bad signal quality of the molded disk). On the other hand, if the sprue aperture is sized very generously to minimize fill and pack problems, then the cooling time required for the sprue's molten plastic to sufficiently solidify to permit its ejection during mold open becomes very long. All known optical disk molds currently use a sprue geometry which is a section cut from a steeply tapered conical solid. (See FIG. 2). Therefore, with this conventional state of arts, there is a direct tradeoff between having the smallest possible sprue for shortest cooling time yet having the largest possible sprue for best product quality and fastest fill and easiest packing.